1. Field of the Invention
The present invention relates to a method, system, and program for moving data among storage units.
2. Description of the Related Art
In a tape library system, a tape controller will perform a reclamation process to improve the utilization of the tape storage units. The reclamation process involves copying active data from one or more tapes having both inactive and active data to fewer tapes that only have active data. The tapes from which the data is copied are then added to a scratch pool of available tapes from which they may be selected and used to store future data. Empty tapes may be returned to a scratch pool or retained for exclusive use of the current pool. This process improves storage capacity utilization by aggregating active data from multiple tapes to a single tape that stores a greater percentage of active data. Reclamation is necessary because as data is modified, older versions of the data on various tapes becomes outdated or inactive. Tapes that have both inactive and active data are not fully utilized because data is written sequentially and inactive data cannot simply be replaced with active data.
A tape is scheduled for reclamation when the amount of active data in a tape reaches a reclamation threshold. In order to optimize tape utilization, the reclamation threshold would be set to a higher level to more frequently consolidate data from tapes with a lower utilization to a single tape with a higher utilization. However, the reclamation process consumes substantial tape library resources to move the data from tape to tape and can affect other tape library operations. For instance, the data movement that occurs during reclamation can interfere with the data movement to tape that occurs in a hierarchical storage management (HSM) system when data is migrated from a faster access storage device, such as an array of hard disk drives, to slower access storage device, such as tape. Setting the reclamation threshold to a higher level to increase tape utilization will increase the frequency of the reclamation process and thereby consume substantial tape library resources and perhaps interfere with other tape library operations, such as data migration when the tape library is used in a hierarchical storage management system.
On the other hand, setting the reclamation threshold lower will reduce the frequency of reclamation because the amount of active data must fall to a relatively low level before reclamation begins. Reducing the frequency of reclamation will consume less tape library resources and minimize interference with other tape library operations, such as data migration from disk to tape. However, reducing the frequency of reclamation allows tapes to remain with a lower storage capacity utilization because reclamation is not performed until the tape storage capacity utilization is at the lower threshold level. If storage capacity utilization is lower, then the data is dispersed across more tapes at a lower capacity utilization.
Thus, there is always a tradeoff of tape library performance and storage capacity utilization that must be considered when determining the reclamation threshold.
For these reasons, there is a need in the art for improved techniques for handling data reclamation in a storage system.